It is known that cellulose is produced as a fibrous form in nature by plants, for example, woody plants such as hardwoods and softwoods, and herbaceous plants such as bamboo and reed, some animals typified by sea squirt, and some fungi typified by acetobacter, and the like. Cellulose molecules having a structure of aggregate in a fibrous form are called a cellulose fiber. In particular, a cellulose fiber having a fiber width of 100 nm or less and an aspect ratio of 100 or more is generally called a cellulose nanofiber (hereinafter referred to as CNF) and has excellent properties such as light weight, high mechanical strength and low coefficient of thermal expansion.
In nature, a CNF does not exist in the form of a single fiber except those produced by some fungi typified by acetobacter. Most of CNFs exist in a firmly aggregated form by interaction typified by hydrogen bonding between CNFs, which form has a micro-size fiber width. Fibers having such a micro-size fiber width exist in a further highly aggregated form.
In a papermaking process, wood is fibrillated by a pulping method typified by a kraft cooking method as one of chemical pulping methods to a state of pulp having a micro-size fiber width, and paper is prepared using the pulp as a starting material. The fiber width of pulp varies depending upon a starting material and is about 5-20 μm, about 20-80 μm and about 5-20 μm with respect to bleached hardwood kraft pulp, bleached softwood kraft pulp and bleached bamboo kraft pulp, respectively.
As described above, such pulp having a micro-size fiber width is an aggregate of single fibers which has a fibrous form and in which CNFs are firmly aggregated by interaction typified by hydrogen bonding, and CNFs as single fibers having a nano-size fiber width are obtained by further advancing fibrillation.
With respect to a CNF molded product, Patent Document 1 discloses a method for preparing a CNF molded product, which comprises steps of bringing a surface of a porous substrate into contact with a liquid containing one or more solvents and one or more polymers, and removing the one or more solvents by means of the porous substrate to bring solid content concentration of the liquid to 4% or more. A CNF molded product is thereby obtained. As the polymer used in this method for preparing a CNF molded product, fibrous polymers of 1 nm to 500 nm in the minor axis direction and 500 nm to 1000 μm in the major axis direction are mentioned.
With a view to providing a material which is lightweight and substantially free from combustion residue at the time of disposal and which has a high mechanical strength. Patent Document 2 discloses a high mechanical strength material having a density of 1.2 g/cm3 to 1.4 g/cm3, a flexural strength of 200 MPa or more and a flexural modulus of 14 GPa or more, which is obtained by hot-pressing cellulose nanofibers having an average fiber diameter of 10 to 100 nm and an average aspect ratio of 1000 or more under high pressure.